1. Field of the Invention
The invention relates to industrial manipulators and, more particularly, the invention is directed to a technique for effecting the control of a robot's movement through at least one degree of freedom.
2. Description of the Prior Art
In virtually every application of an industrial manipulator or robot, it is necessary to exercise a specific level of control to the ranges of angular freedom, the working strokes and the like, and for reasons of safety, standing obstacles, overload damage prevention and various other factors. Control of the range of movements available to a given manipulator can be a critical consideration in flexible manufacturing cell applications. Typically, a flexible manufacturing cell incorporates two or more structurally independent industrial manipulators which are integrated for the purpose of a coordinated work effort. Unless exact control of the movement of each manipulator is assured, self-inflicted damage to one or more of the several manipulators in a cell is a serious possibility. While such problems may not arise when the working range of a single industrial manipulator is limited to an individual degree of freedom which can be fully traversed without encountering any obstacles, such a simple situation is normally not the case.
A typical solution utilized to limit the working strokes, such as axially based rotational displacement and other movements, is by means of mechanical stops, or the like. However, various problems are commonly associated with mechanical stops. In the first place, the mechanical stops must be very rugged in order to withstand the force or shearing moment generated by industrial manipulators designed for heavy duty applications, otherwise the stops could be either moved or even destroyed by the movements of the industrial manipulators. An additional problem arises with industrial robots having several degrees of freedom in their normal operations because the permissible angles of rotation or movement in any given degree of freedom can be effected by such motions in one or more of the remaining degrees of freedom. If various types of job assignments are to be carried out by a single industrial manipulator, as is the case with the several manipulators in a flexibly, arrangeable or programmable manufacturing cell or installation, the mechanical stops must be moved quite frequently. Repositioning the mechanical stops is not only a taxing and time-consuming operation, but it is also an operation which can involve considerable safety risks. In a flexible manufacturing cell, the problem becomes all the more troublesome, the more often the manipulator's controlling program is changed.
Similar problems are also encountered when an operator is "setting-up" or defining the limits of travel for a given degree of freedom in an industrial manipulator. By "setting-up" what is meant is the procedure whereby the agreement between the mechanical null or zero position of one's drive mechanism's axis and the null position of a then-assigned transmitter is established. By using absolute value transmitters, this measure is required only once, as for example, upon the installation or the replacement of a transmitter. On the other hand, the aforedescribed measure is required after each re-insertion of the control if use is made of periodically or cyclically operating measured value transmitters which, hereinafter, will be designated as relative measured value transmitters.
Since measured value transmitters (sensors) which have high resolving power, which are accurate and which have the absolute value form of construction are extremely expensive and of a rather large size, use is predominantly made of the smaller and more cost-favorable relative transducers, especially incremental transmitters. However, the use of such incremental transmitters requires that setting-up becomes a task to be carried out relatively frequently.
The free area of movement, which, in many installation, is restricted by external conditions as described above, frequently does not permit the use of the generally common null mark of the transmitter employed to produce the desired reference setting since, for example, this angular setting could lie entirely in a region which cannot be traversed by the robot's arm due to the fact than an obstacle is present. In this case, in conventional stepups, the transmitters must be released and--departing from the normal setting--the transmitter must be so rotated that alignment is possible in a traversable region.